**Core Concept**
The left ureter receives its blood supply from multiple sources, primarily the lower third from the uterine artery and the upper two-thirds from the renal and common iliac arteries, with anastomoses between these vessels.

**Why the Correct Answer is Right**
The correct answer is the uterine artery, which supplies the lower third of the left ureter. This is due to the close anatomical relationship between the left ureter and the uterine artery as they course through the pelvic cavity. The uterine artery originates from the internal iliac artery and travels medially, where it forms an anastomosis with the middle rectal artery and the vaginal artery. This anastomosis with the uterine artery provides the primary blood supply to the lower third of the left ureter.

**Why Each Wrong Option is Incorrect**
* **Option B:** The inferior vesical artery primarily supplies the bladder and is not the primary source of blood supply to the left ureter.
* **Option C:** The testicular artery is involved in supplying the testes and is not related to the blood supply of the left ureter.
* **Option D:** The common iliac artery gives rise to the renal and internal iliac arteries, but it is not the direct source of blood supply to the left ureter.
* **Option E:** The external femoral artery is not directly related to the blood supply of the left ureter, as it primarily supplies the lower limb.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the blood supply to the ureters can vary significantly between individuals, and knowledge of these anatomical variations is crucial for surgical procedures and managing ureteral injuries.

**Correct Answer:** A. Uterine artery